The present disclosure relates to a semiconductor laser device, and more particularly to a semiconductor laser device with external resonator, which includes a semiconductor light-emitting element which does not oscillate light by itself, and an external resonator configured to oscillate light emitted from the semiconductor light-emitting element.
Conventionally, semiconductor laser devices are widely used as a recording light source for various image recording devices and a measuring light source for various measurement devices, for example. As one type of such semiconductor laser devices, semiconductor laser devices that include a semiconductor laser and an external resonator configured to select the wavelength of laser light emitted from the semiconductor laser to be returned to the semiconductor laser are known, as taught in Japanese Unexamined Patent Publication No. 2001-242500 (Patent Document 1), for example. In this type of semiconductor laser devices, a resonator structure of the semiconductor laser that oscillates light by itself and the external resonator form a so-called composite resonator.
In some of the above-described type of semiconductor laser devices, the wavelength of the laser light within a desired wavelength range is selected using a wavelength control element, which is formed by a narrow-band band pass filter, or the like, and is disposed in the optical path of the external resonator. Further, with this type of semiconductor laser devices, fast modulation drive of the semiconductor laser is performed by supplying a drive current with a high-frequency wave superimposed thereon, for example, to the semiconductor laser. Patent Document 1 also teaches that the wavelength is selected using a wavelength control element (which is referred to as “wavelength selecting element” in Patent Document 1), and that fast modulation drive of the semiconductor laser is performed.
On the other hand, as another type of semiconductor laser devices with external resonator; a semiconductor laser device that includes a semiconductor light-emitting element which does not oscillate light by itself, and an external resonator configured to oscillate light emitted from the semiconductor light-emitting element is known, as taught in Japanese Unexamined Patent Publication No. H11 (1999)-17286 (Patent Document 2), for example. It should be noted that the semiconductor light-emitting element is referred to as “laser diode” in Patent Document 2.
Also, in this type of semiconductor laser devices, the wavelength of laser light within a desired wavelength range to be oscillated is selected using a wavelength control element which is formed by a narrow-band band pass filter, or the like, and is disposed in the optical path of the external resonator. Patent Document 2 also teaches about the selection of the wavelength.
Patent Document 1 teaches that, in the case where the wavelength control element is disposed in the external resonator that forms the composite resonator and the fast modulation drive of the semiconductor laser is performed, the longitudinal mode of the laser light emitted from the semiconductor laser device becomes multi-mode. In contrast, in the case where the wavelength control element is disposed in the external resonator, which is the only resonator, as taught in Patent Document 2, it is believed that the longitudinal mode of the laser light emitted from the semiconductor laser device usually becomes single mode. Actually, Patent Document 2 teaches that the wavelength of the laser light can be varied by rotating the band pass filter; however, it does not particularly teach or suggest that multiple wavelengths are present simultaneously.